Synthetic resin molded articles, such as polymethyl methacrylate resins, polymethacrylimide resins, polycarbonate resins, polystyrene resins, polyolefin resins, and the like, are lighter in weight and superior in impact-resistance compared to glass articles. Because they exhibit a variety of advantages, such as good clarity and ease of molding and processing, they have recently been used in a variety of fields, such as automotive window panes, headlights, tail lights, as well as architectural window panes, highway soundproof barrier walls, and the like.
On the other hand, because their surface lacks abrasion resistance, these synthetic resin molded articles tend to suffer some surface-damage when coming into contact with other hand articles or sandy dust and through friction and scratching by such articles. Since the surface damage substantially reduces the commercial value of such molded articles or causes the product to be unusable within short periods, improvement of surface abrasion resistance has been much needed. Particularly, when they are used in the aforementioned automotive materials, weathering resistance is also another important requirement.
Many studies have been made of methods to overcome the deficiencies of these synthetic resin molded articles; for example, there is a method (U.S. Pat. No. 4,006,271) that calls for applying a coating agent comprising a partial hydrolysis condensate of a silane compound, mainly based on an alkyltrialkoxy silane, and colloidal silica, then followed by heat treating, and thereby generating a cross-linked coating to improve the wear resistance..
Said process provides plastic molded articles with a high degree of abrasion resistance, but these articles are not satisfactorily in durability or weathering resistance, and the coating, which is based on a coating agent comprising a partial hydrolysis condensate of the above silane compound and colloidal silica, exhibits insufficient adhesion.
A method has been developed to provide a coating with improved adhesion by a prior priming, with an acrylic or silicone or the like, of a plastic molded article for improved adhesion between the molded article and the primer, and then coating the primed layer with the aforementioned coating, thereby providing improved adhesion between said primer layer and the coated layer. However, this coating method involves complicated process steps and does not provide satisfactory properties. Since the coating-curing time is long, the process is economically disadvantageous and also poor in productivity.
A method overcoming such a deficiency is disclosed in Japanese Patent Kohyo Publication No. Sho57-500984 (U.S. Pat. No. 4,348,462) involving an ultraviolet-curable coating agent comprising colloidal silica and a small amount of an alkoxysilane having a methacryloyloxy group or a glycidyl group, wherein a non-silyl acrylate being the main component, thereby improving the adhesion to the surface of the plastic molded article, as well as a method of coating a plastic molded article surface with this coating agent, followed by irradiating it with ultraviolet light for curing, thereby generating a plastic molded article with a wear resistant coating. Coating compositions, which are essentially free of any non-polymerizable organic solvent, comprising colloidal silica, small amounts of a hydrolysis product of a silylacrylate, photo-polymerization initiators, and polyfunctional acrylates being as main components, have also been disclosed in Japanese Patent Laid-open Publication No. Sho58-1756 (U.S. Pat. Nos. 4,455,205, 4,486,504 and 4,491,508); ultraviolet-curable coating compositions containing a small amount of a colloidal silica which has been chemically modified with a radical polymerizable silane, comprising as a main component a polyfunctional acrylate and a polyfunctional urethane acrylate having a specific structure, have been disclosed in Japanese Patent Laid-open Publication No. Hei4-214743 (U.S. Pat. Nos. 5,258,225 and 5,426,131).
Compositions generating these abrasion resistant coatings have provided excellent adhesion to substrates with no primer treatment and allowed the coating-curing time to be shortened, but they have tended to make the coating turbid, making it difficult to obtain a completely clear cured coating, and have shown an unsatisfactory balance with durability such as weather resistance, abrasion resistance, and the like.
The present inventors continued their analysis of the causes for the above problems and discovered that the condensation reaction between the colloidal silica and the radical polymerizable silane compound has been insufficient, causing the resultant chemically modified colloidal silica to exhibit an inferior compatibility with the polyfunctional acrylate, and proposed a photo-polymerizable coating composition as disclosed in Japanese Patent Laid-open Publication No. Hei3-275769, which composition comprised a chemically modified colloidal silica with (meth)acryloyloxy silane upon sufficiently advanced conversion in the condensation reactor, along with bis(acryloyloxyethyl)hydroxyethyl isocyanurate,and a photo-polymerization initiator.
The cured coatings generated from the above coating composition had good features in that they were free of turbidity and clear, with good optical properties, and with relatively good abrasion resistance under the Taber abrasion test, steel wool test, and the like, as well as with good weather resistance, but none have yet provided an ultraviolet-curable covering material having completely satisfactory properties because of a crack formation problem in a heat cycle test.
After a diligent analysis of the causes for the above problem, the present inventors discovered that allowing the condensation reaction between the colloidal silica and the (meth)acryloyloxy silane compound to proceed fully has improved, to some extent, the compatibility of the chemically modified colloidal silica with the bis(acryloyloxyethyl)hydroxyethyl isocyanate, but their compatibility has still not been enough, making it impossible to sufficiently suppress the phase separation of these two components in the process of generating a cured coating, which was a major cause of the above deficiency.